What medication would you prescribe for a 33-year-old male patient with an open tibial shaft comminuted fracture, who is experiencing watery diarrhea while on cefazolin (Cefazolin) and gentamicin (Gentamicin), with options including vancomycin (Vancomycin), clindamycin (Clindamycin), or fluoroquinolones (Fluoroquinolones)?

Treatment for Antibiotic-Associated Diarrhea in a Patient with Open Tibial Fracture

Vancomycin is the most appropriate medication for this patient experiencing watery diarrhea while on cefazolin and gentamicin therapy for an open tibial shaft comminuted fracture. 1

Rationale for Vancomycin Selection

Antibiotic-associated diarrhea in this patient is most likely caused by Clostridioides difficile infection (CDI), which commonly occurs with the use of cephalosporins (like cefazolin) and aminoglycosides (like gentamicin). The clinical presentation of watery diarrhea while on antibiotics strongly suggests CDI.

According to treatment guidelines for CDI:

• Vancomycin 125 mg four times daily orally for 10 days is recommended for CDI treatment 1
• Vancomycin has demonstrated superior clinical success rates (80-81%) in treating CDI 2
• For non-severe CDI, vancomycin 125 mg four times daily orally for 10 days is a first-line option 1

Why Not the Other Options?

1. Clindamycin: Not recommended because:

   • Clindamycin is actually one of the antibiotics most frequently associated with causing CDI 3, 4
   • Using clindamycin could potentially worsen the patient's diarrhea
   • No guidelines support clindamycin for treatment of antibiotic-associated diarrhea

2. Fluoroquinolones: Not recommended because:

   • Fluoroquinolones are also associated with causing CDI 1
   • While fluoroquinolones (ciprofloxacin, levofloxacin) are effective for treating travelers' diarrhea and certain bacterial causes of diarrhea 5, 6, they are not first-line for CDI
   • Using fluoroquinolones could potentially worsen CDI

Management Algorithm

1. Confirm diagnosis:

   • Test stool for C. difficile toxin (enzyme immunoassay is practical and reasonably accurate) 3
   • Assess severity of diarrhea (frequency, consistency, presence of blood)
   • Check for signs of colitis (fever, abdominal cramps, leukocytosis)

2. Initial treatment:

   • Start oral vancomycin 125 mg four times daily for 10 days 1
   • If oral administration is not possible, consider metronidazole 500 mg three times daily intravenously 1

3. Supportive care:

   • Provide fluid resuscitation and electrolyte replacement 1
   • Consider albumin supplementation if severe hypoalbuminemia (<2 g/dL) is present 1

4. Monitor response:

   • Assess for resolution of diarrhea (median time to resolution: 4-5 days) 2
   • Watch for recurrence after treatment completion (occurs in 18-23% of cases) 2

Important Considerations

• Fracture management: Continue appropriate antibiotic coverage for the open tibial fracture, as infection prevention remains critical 7, 8

• Recurrence risk: Patients over 65 years may have longer time to resolution (4-6 days) and higher recurrence risk 2

• Severe CDI: If the patient develops severe CDI (≥10 unformed bowel movements/day or WBC ≥15,000/mm³), vancomycin remains the appropriate choice 1

• Caution: Avoid anti-motility agents like loperamide in suspected CDI as they may worsen outcomes by delaying toxin clearance

By selecting vancomycin, you are addressing the most likely cause of the patient's diarrhea while ensuring continued appropriate management of the open tibial fracture.